Anomalous Swelling Behavior of Poly(N-vinylimidazole)-l-Poly(tetrahydrofuran) Amphiphilic Conetwork in Water Studied by Solid-State NMR and Positron Annihilation Lifetime Spectroscopy

Poly(N-vinylimidazole) homopolymer (PVIm) and poly(N-vinylimidazole)-l-poly(tetrahydrofuran) (PVIm-l-PTHF), a novel amphiphilic polymer conetwork (APCN), were synthesized to compare their solid state structure and investigate the swelling behavior of this unique conetwork in water. A short-range ordered structure stabilized by second-order interactions between the imidazole pendant groups was found in the PVIm homopolymer and in the PVIm phase of the dry conetwork as revealed by solid-state NMR C-13 cross-polarization magic angle spinning (CP MAS) and two-dimensional H-1-C-13 frequency-switched Lee-Goldburg (FSLG) HETCOR measurements. With increasing swelling ratio, structural and conformational changes were recognized in the hydrophilic PVIm phase of the APCN. In the kinetic swelling study, unexpectedly, four different swelling ranges were identified by gravimetric measurements, solid-state NMR methods, and positron annihilation lifetime (PAL) spectroscopy before the APCN reached its equilibrium swelling state. In the first period, which takes place in several minutes, the ordered structure disintegrates in the PVIm phase and the water uptake is relatively slow. This structural realignment is followed by the main course of water uptake governed by Fickian diffusion in the second stage. Close to the equilibrium swelling ratio, the swelling curve becomes nonmonotonic caused by a realignment of main chains of the hydrophilic phase in the third stage of swelling. Thus, by the unique combination of conventional swelling kinetics and solid-state NMR as well as PAL spectroscopies for investigating the aqueous swelling of the PVIm-l-PTHF amphiphilic polymeric conetwork, it was revealed that unexpected noncontinuous swelling occurs which is due to structural changes of the PVIm component in the conetwork in the course of this process.